1. Field of the Invention
This invention relates to a method and an apparatus for magnetically recording and/or reproducing video, audio or other information signals, and is suitably applied to a digital VTR which is also capable of reproducing a signal recorded with a different format, for example, the forms used in an analog VTR.
2. Description of the Related Art
Heretofore, when a video tape recorder is employed for recording and reproducing a new recording format in which the track length of the helical recording track is extended compared with the existing recording format, such differences in track length has been coped with by not changing the drum diameter but by changing the wrap angle of the magnetic tape on the drum.
For instance, as shown in FIG. 1, in the case where a new recording format is provided by eliminating one of the longitudinal sound channels TR.sub.A1 and extending the track length of a helical recording track TR from the existing 180.degree. switching recording format, a wrap angle of the magnetic tape TP on the drum is increased to 180.degree.+.alpha. and thus the magnetic tape TP of new recording format can be scanned by heads of the existing drum, that is, without changing the drum diameter or the positions of the heads thereon.
However, if, as described above, we try to cope with the increased track length by increasing the wrap angle, the reproduced signals obtained from two magnetic heads which are in opposing 180 degree relation overlap in time. It becomes impossible to transmit such reproduced signals through a common system and it becomes necessary to have independent rotary transmitters for the drums and subsequent signal processing circuits respectively, and thus, the space factor becomes inefficient and the cost increases.
Also, in the case where the wrap angle is increased as shown in FIG. 2, that is, by changing the path of the magnetic tape TP wrapped around the drum at the exit from the drum DR, if the wrap angle is overly increased, there has been a problem that simple U loading of this tape is not possible in view of the reduced mechanical space between the tape runs to and from the drum.
In practice, in case of reproducing video signals from the magnetic tape while utilizing the drum described above, there are 525 (525/60 systems) or 625 (625/50 systems) horizontal synchronizing signals reproduced during each full turn of the drum. Therefore, pulses synchronized with the reproduced horizontal synchronizing signals and pulses synchronized with the standard horizontal synchronizing signals can be counted and if there is coincidence between the numbers of pulses and horizontal synchronizing signals counted during a full turn of the drum, it can be judged to be normal reproduction.
Furthermore, if the number of reproduced horizontal synchronizing signals counted at the end of a full turn of the drum is an integer, the pulses continue as horizontal synchronizing signals, and it is possible to lock vertical synchronizing signals from the top if the skew is within .+-.1/2H as the reproducing field continues further.
It is possible to perform interpolation by counting pulses which are synchronized with horizontal synchronizing signals from the preceding vertical synchronizing signals when the vertical synchronizing signals can not be detected. However, an attempt is made to change the drum diameter in order to solve the problem presented by a change in track length, it is not easy to perform these processes unless there is an integer relation between the number of horizontal synchronizing signals in the standard synchronizing signal system and in the different format of the reproducing system.